Site-specific PEGylation of native disulfide bonds in therapeutic proteins

Protein–polymer therapeutics: a macromolecular perspective

The development of protein–polymer hybrids emerged several decades ago with the vision that their synergistic combination will offer macromolecular hybrids with manifold features to succeed as the next generation therapeutics.

Site-specific PEGylation of proteins: recent developments

The attachment of linear polyethylene glycol (PEG) to peptides and proteins for their stabilization for in vivo applications is a milestone in pharmaceutical research and protein-drug development. However, conventional methods often lead to heterogeneous PEGylation mixtures with reduced protein activity. Current synthetic efforts aim to provide site-specific approaches by chemoselective targeting of canonical and noncanonical amino acids and to improve the PEG architecture. This synopsis highlights recent work in this area, which also resulted in improved pharmacokinetics of peptide and protein therapeutics.

A disulfide intercalator toolbox for the site-directed modification of polypeptides

A disulfide intercalator toolbox was developed for site-specific attachment of a broad variety of functional groups to proteins or peptides under mild, physiological conditions. The peptide hormone somatostatin (SST) served as model compound for intercalation into the available disulfide functionalization schemes starting from the intercalator or the reactive SST precursor before or after bioconjugation. A tetrazole-SST derivative was obtained that undergoes photoinduced cycloaddition in mammalian cells, which was monitored by live-cell imaging.

Therapeutic protein-polymer conjugates: advancing beyond PEGylation

Protein-polymer conjugates are widely used as therapeutics. All Food and Drug Administration (FDA)-approved protein conjugates are covalently linked to poly(ethylene glycol) (PEG). These PEGylated drugs have longer half-lives in the bloodstream, leading to less frequent dosing, which is a significant advantage for patients. However, there are some potential drawbacks to PEG that are driving the development of alternatives. Polymers that display enhanced pharmacokinetic properties along with additional advantages such as improved stability or degradability will be important to advance the field of protein therapeutics. This perspective presents a summary of protein-PEG conjugates for therapeutic use and alternative technologies in various stages of development as well as suggestions for future directions. Established methods of producing protein-PEG conjugates and new approaches utilizing controlled radical polymerization are also covered.

Agile delivery of protein therapeutics to CNS

A variety of therapeutic proteins have shown potential to treat central nervous system (CNS) disorders. Challenge to deliver these protein molecules to the brain is well known. Proteins administered through parenteral routes are often excluded from the brain because of their poor bioavailability and the existence of the blood-brain barrier (BBB). Barriers also exist to proteins administered through non-parenteral routes that bypass the BBB. Several strategies have shown promise in delivering proteins to the brain. This review, first, describes the physiology and pathology of the BBB that underscore the rationale and needs of each strategy to be applied. Second, major classes of protein therapeutics along with some key factors that affect their delivery outcomes are presented. Third, different routes of protein administration (parenteral, central intracerebroventricular and intraparenchymal, intranasal and intrathecal) are discussed along with key barriers to CNS delivery associated with each route. Finally, current delivery strategies involving chemical modification of proteins and use of particle-based carriers are overviewed using examples from literature and our own work. Whereas most of these studies are in the early stage, some provide proof of mechanism of increased protein delivery to the brain in relevant models of CNS diseases, while in few cases proof of concept had been attained in clinical studies. This review will be useful to broad audience of students, academicians and industry professionals who consider critical issues of protein delivery to the brain and aim developing and studying effective brain delivery systems for protein therapeutics.

Derivatization of antibody Fab fragments: a designer enzyme for native protein modification

Bioconjugates, such as antibody-drug conjugates, have gained recent attention because of their increasing use in therapeutic and diagnostic applications. Commonly used conjugation reactions based upon chemoselective reagents exhibit a number of drawbacks: most of these reactions lack regio- and stereospecificity, thus resulting in loss of protein functionality due to random modifications. Enzymes provide an obvious solution to this problem, but the intrinsic (natural) substrate specificities of existing enzymes pose severe limitations to the kind of modifications that can be introduced. Here we describe the application of the novel trypsin variant trypsiligase for site-specific modification of the C terminus of a Fab antibody fragment via a stable peptide bond. The suitability of this designed biocatalyst was demonstrated by coupling the Her2-specific Fab to artificial functionalities of either therapeutic (PEG) or diagnostic (fluorescein) relevance. In both cases we obtained homogeneously modified Fab products bearing the artificial functionality exclusively at the desired position.

Site-specific functionalization of proteins and their applications to therapeutic antibodies

Protein modifications are often required to study structure and function relationships. Instead of the random labeling of lysine residues, methods have been developed to (sequence) specific label proteins. Next to chemical modifications, tools to integrate new chemical groups for bioorthogonal reactions have been applied. Alternatively, proteins can also be selectively modified by enzymes. Herein we review the methods available for site-specific modification of proteins and their applications for therapeutic antibodies.

A new reagent for stable thiol-specific conjugation

Many clinically used protein therapeutics are modified to increase their efficacy. Example modifications include the conjugation of cytotoxic drugs to monoclonal antibodies or poly(ethylene glycol) (PEG) to proteins and peptides. Monothiol-specific conjugation can be efficient and is often accomplished using maleimide-based reagents. However, maleimide derived conjugates are known to be susceptible to exchange reactions with endogenous proteins. To address this limitation in stability, we have developed PEG-mono-sulfone 3, which is a latently reactive, monothiol selective conjugation reagent. Comparative reactions with PEG-maleimide and other common thiol-selective PEGylation reagents including vinyl sulfone, acrylate, and halo-acetamides show that PEG-mono-sulfone 3 undergoes more efficient conjugation under mild reaction conditions. Due to the latent reactivity of PEG-mono-sulfone 3, its reactivity can be tailored and, once conjugated, the electron-withdrawing ketone is easily reduced under mild conditions to prevent undesirable deconjugation and exchange reactions from occurring. We describe a comparative stability study demonstrating a PEG-maleimide conjugate to be more labile to deconjugation than the corresponding conjugate obtained using PEG-mono-sulfone 3.

RAFT aqueous dispersion polymerization yields poly(ethylene glycol)-based diblock copolymer nano-objects with predictable single phase morphologies

A poly(ethylene glycol) (PEG) macromolecular chain transfer agent (macro-CTA) is prepared in high yield (>95%) with 97% dithiobenzoate chain-end functionality in a three-step synthesis starting from a monohydroxy PEG₁₁₃ precursor. This PEG₁₁₃-dithiobenzoate is then used for the reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate (HPMA). Polymerizations conducted under optimized conditions at 50 °C led to high conversions as judged by ¹H NMR spectroscopy and relatively low diblock copolymer polydispersities (M_w/M_n < 1.25) as judged by GPC. The latter technique also indicated good blocking efficiencies, since there was minimal PEG₁₁₃ macro-CTA contamination. Systematic variation of the mean degree of polymerization of the core-forming PHPMA block allowed PEG₁₁₃-PHPMA_x diblock copolymer spheres, worms, or vesicles to be prepared at up to 17.5% w/w solids, as judged by dynamic light scattering and transmission electron microscopy studies. Small-angle X-ray scattering (SAXS) analysis revealed that more exotic oligolamellar vesicles were observed at 20% w/w solids when targeting highly asymmetric diblock compositions. Detailed analysis of SAXS curves indicated that the mean number of membranes per oligolamellar vesicle is approximately three. A PEG₁₁₃-PHPMA_x phase diagram was constructed to enable the reproducible targeting of pure phases, as opposed to mixed morphologies (e.g., spheres plus worms or worms plus vesicles). This new RAFT PISA formulation is expected to be important for the rational and efficient synthesis of a wide range of biocompatible, thermo-responsive PEGylated diblock copolymer nano-objects for various biomedical applications.

Building better drugs: developing and regulating engineered therapeutic proteins

Most native proteins do not make optimal drugs and thus a second- and third-generation of therapeutic proteins, which have been engineered to improve product attributes or to enhance process characteristics, are rapidly becoming the norm. There has been unprecedented progress, during the past decade, in the development of platform technologies that further these ends. Although the advantages of engineered therapeutic proteins are considerable, the alterations can affect the safety and efficacy of the drugs. We discuss both the key technological innovations with respect to engineered therapeutic proteins and advancements in the underlying basic science. The latter would permit the design of science-based criteria for the prediction and assessment of potential risks and the development of appropriate risk management plans. This in turn holds promise for more predictable criteria for the licensure of a class of products that are extremely challenging to develop but represent an increasingly important component of modern medical practice.

Homogeneous antibody fragment conjugation by disulfide bridging introduces 'spinostics'

A major obstacle to the efficient production of antibody conjugates for therapy and diagnosis is the non-ideal performance of commonly used chemical methods for the attachment of effector-molecules to the antibody of interest. Here we demonstrate that this limitation can be simply addressed using 3,4-substituted maleimides to bridge and thus functionalize disulfide bonds to generate homogeneous antibody conjugates. This one-step conjugation reaction is fast, site-specific, quantitative and generates products with full binding activity, good plasma stability and the desired functional properties. Furthermore, the rigid nature of this modification by disulfide bridging enables the successful detection of antigen with a spin labeled antibody fragment by continuous-wave electron paramagnetic resonance (cw-EPR), which we report here for the first time. Antigen detection is concentration dependent, observable in human blood and allows the discrimination of fragments with different binding affinity. We envisage broad potential for antibody based in-solution diagnostic methods by EPR or ‘spinostics'.

Design, synthesis, and application of stimulus-sensing biohybrid hydrogels

A key feature of any living system is the ability to sense and react to the environmental stimuli. The biochemical characterization of the underlying biological sensors combined with advances in polymer chemistry has enabled the development of stimulus-sensitive biohybrid materials that translate most diverse chemical and biological input into a precise change in material properties. In this review article, we first describe synthesis strategies of how biological and chemical polymers can functionally be interconnected. We then provide a comprehensive overview of how the different properties of biological sensor molecules such as competitive target binding and allosteric modulation can be harnessed to develop responsive materials with applications in tissue engineering and drug delivery.

Oral drug delivery systems using chemical conjugates or physical complexes

Oral delivery of therapeutics is extremely challenging. The digestive system is designed in a way that naturally allows the degradation of proteins or peptides into small molecules prior to absorption. For systemic absorption, the intact drug molecules must traverse the impending harsh gastrointestinal environment. Technologies, such as enteric coating, with oral dosage formulation strategies have successfully provided the protection of non-peptide based therapeutics against the harsh, acidic condition of the stomach. However, these technologies showed limited success on the protection of therapeutic proteins and peptides. Importantly, inherent permeability coefficient of the therapeutics is still a major problem that has remained unresolved for decades. Addressing this issue in the context, we summarize the strategies that are developed in enhancing the intestinal permeability of a drug molecule either by modifying the intestinal epithelium or by modifying the drug itself. These modifications have been pursued by using a group of molecules that can be conjugated to the drug molecule to alter the cell permeability of the drug or mixed with the drug molecule to alter the epithelial barrier function, in order to achieve the effective drug permeation. This article will address the current trends and future perspectives of the oral delivery strategies.

Tailoring polymeric micelles to optimize delivery to solid tumors

Block copolymer micelles have shown great potential in drug delivery systems, not only for overcoming the drawbacks of small agents such as water insolubility and wide distribution in normal tissues, but also for avoiding traditional nanoparticle formulation shortcomings, including in vivo instability and fast clearance from the blood. However, for translating micellar formulations to clinical practice, it is essential to overcome the many in vivo obstacles. Surmounting these barriers strongly depends on micellar physicochemical properties, which can be further optimized by the unique physiological aspects of solid tumors such as low pH, high temperature and the presence of abnormal vessels. Herein, based on the Flory parameter and scaling theory, the fundamental mechanisms and correlations in vitro/in vivo between self assembly, drug loading and release, stability, intracellular delivery and in vivo distribution, as well as micellar composition, size and microstructural tailoring are systematically revisited. The methods for enhancing micellar performance in solid tumors were consequently proposed through well-defined core-corona structure tailoring.

Bioconjugation Using Thiols: Old Chemistry Rediscovered to Connect Polymers with Nature's Building Blocks

Various pathways to bioconjugates based on thiol chemistry are discussed. Thiol-halogeno, thiol-parafluoro, thiol-ene, thiol-yne, thiol-vinylsulfone and thiol-vinyl sulfone, thiol-maleimide, thiol-bisulfone, and thiol-pyridyl disulfide are well-established synthetic routes discovered in recent years as tools to marry polymers with biomolecules such as carbohydrates, proteins, peptide, DNA, antibodies, or other building blocks from nature.

Comparative binding of disulfide-bridged PEG-Fabs

Protein PEGylation is the most clinically validated method to improve the efficacy of protein-based medicines. Antibody fragments such as Fabs display rapid clearance from blood circulation and therefore are good candidates for PEGylation. We have developed PEG-bis-sulfone reagents 1 that can selectively alkylate both sulfurs derived from a native disulfide. Using PEG-bis-sulfone reagents 1, conjugation of PEG specifically targets the disulfide distal to the binding region of the Fab (Scheme 2 ). PEG-bis-sulfone reagents 1 (10-40 kDa) were used to generate the corresponding PEG-mono-sulfones 2 that underwent essentially quantitative conjugation to give the PEG-Fab product 4. Four Fabs were PEGylated: Fab(beva), Fab'(beva), Fab(rani), and Fab(trast). Proteolytic digestion of bevacizumab with papain gave Fab(beva), while digestion of bevacizumab with IdeS gave F(ab')(2-beva), which after reaction with DTT and PEG-mono-sulfone 2 gave PEG(2)-Fab'(beva). Ranibizumab, which is a clinically used Fab, was directly PEGylated to give PEG-Fab(rani). Trastuzumab was proteolytically digested with papain, and its corresponding Fab was PEGylated to give PEG-Fab(trast). Purification of the PEGylated Fabs was accomplished by a single ion exchange chromatography step to give pure PEG-Fab products as determined by silver-stained SDS-PAGE. No loss of PEG was detected post conjugation. A comparative binding study by SPR using Biacore with low ligand immobilization density was conducted using (i) VEGF(165) for the bevacizumab and ranibizumab derived products or (ii) HER2 for the trastuzumab derived products. VEGF(165) is a dimeric ligand with two binding sites for bevacizumab. HER2 has one domain for the binding of trastuzumab. Binding studies with PEG-Fab(beva) indicated that the apparent affinity was 2-fold less compared to the unPEGylated Fab(beva). Binding properties of the PEG-Fab(beva) products appeared to be independent of PEG molecular weight. Site-specific conjugation of two PEG molecules gave PEG(2×20)-Fab'(beva), whose apparent binding affinity was similar to that observed for PEG-Fab(beva) derivatives. The k(d) values were similar to those of the unPEGylated Fab(beva); hence, once bound, PEG-Fab(beva) remained bound to the same degree as Fab(beva). Biacore analysis indicated that both Fab(rani) and PEG(20)-Fab(rani) did not dissociate from the immobilized VEGF at 25 °C, but ELISA using immobilized VEGF showed 2-fold less apparent binding affinity for PEG(20)-Fab(rani) compared to the unPEGylated Fab(rani). Additionally, the apparent binding affinities for trastuzumab and Fab(trast) were comparable by both Biacore and ELISA. Biacore results suggested that trastuzumab had a slower association rate compared to Fab(trast); however, both molecules displayed the same apparent binding affinity. This could have been due to enhanced rebinding effects of trastuzumab, as it is a bivalent molecule. Analogous to PEG-Fab(beva) products, PEG(20)-Fab(trast) displayed 2-fold lower binding compared to Fab(trast) when evaluated by ELISA. The variations in the apparent affinity for the PEGylated Fab variants were all related to the differences in the association rates (k(a)) rather than the dissociation rates (k(d)). We have shown that (i) Fabs are well-matched for site-specific PEGylation with our bis-alkylation PEG reagents, (ii) PEGylated Fabs display only a 2-fold reduction in apparent affinity without any change in the dissociation rate, and (iii) the apparent binding rates and affinities remain constant as the PEG molecular weight is varied.

How do proteins unfold upon adsorption on nanoparticle surfaces?

Owing to their many outstanding features, such as small size, large surface area, and cell penetration ability, nanoparticles have been increasingly used in medicine and biomaterials as drug carriers and diagnostic or therapeutic agents. However, our understanding of the interactions of biological entities, especially proteins, with nanoparticles is far behind the explosive development of nanotechnology. In typical protein-nanoparticle interactions, two processes (i.e., surface binding and conformational changes in proteins) are intermingled with each other and have not yet been quantitatively described. Here, by using a stopped-flow fast mixing technique, we were able to shed light on the kinetics of the adsorption-induced protein unfolding on nanoparticle surfaces in detail. We observed a biphasic denaturation behavior of protein GB1 on latex nanoparticle surfaces. Such kinetics can be adequately described by a fast equilibrium adsorption followed by a slow reversible unfolding of GB1. On the basis of this model, we quantitatively measured all rate constants that are involved in this process, from which the free-energy profile is constructed. This allows us to evaluate the effects of environmental factors, such as pH and ionic strength, on both the adsorption and the conformational change in GB1 on the latex nanoparticle surface. These studies provide a general physical picture of the adsorption-induced unfolding of proteins on nanoparticle surfaces and a quantitative description of the energetics of each transition. We anticipate that it will greatly advance our current understanding of protein-nanoparticle interactions and will be helpful for the rational control of such interactions in biomedical applications.

Site-specific PEGylation of therapeutic proteins via optimization of both accessible reactive amino acid residues and PEG derivatives

Modification of accessible amino acid residues with poly(ethylene glycol) [PEG] is a widely used technique for formulating therapeutic proteins. In practice, site-specific PEGylation of all selected/engineered accessible nonessential reactive residues of therapeutic proteins with common activated PEG derivatives is a promising strategy to concomitantly improve pharmacokinetics, allow retention of activity, alleviate immunogenicity, and avoid modification isomers. Specifically, through molecular engineering of a therapeutic protein, accessible essential residues reactive to an activated PEG derivative are substituted with unreactive residues provided that protein activity is retained, and a limited number of accessible nonessential reactive residues with optimized distributions are selected/introduced. Subsequently, all accessible nonessential reactive residues are completely PEGylated with the activated PEG derivative in great excess. Branched PEG derivatives containing new PEG chains with negligible metabolic toxicity are more desirable for site-specific PEGylation. Accordingly, for the successful formulation of therapeutic proteins, optimization of the number and distributions of accessible nonessential reactive residues via molecular engineering can be integrated with the design of large-sized PEG derivatives to achieve site-specific PEGylation of all selected/engineered accessible reactive residues.

Site-selective lysine modification of native proteins and peptides via kinetically controlled labeling

The site-selective modification of the proteins RNase A, lysozyme C, and the peptide hormone somatostatin is presented via a kinetically controlled labeling approach. A single lysine residue on the surface of these biomolecules reacts with an activated biotinylation reagent at mild conditions, physiological pH, and at RT in a high yield of over 90%. In addition, fast reaction speed, quick and easy purification, as well as low reaction temperatures are particularly attractive for labeling sensitive peptides and proteins. Furthermore, the multifunctional bioorthogonal bioconjugation reagent (19) has been achieved allowing the site-selective incorporation of a single ethynyl group. The introduced ethynyl group is accessible for, e.g., click chemistry as demonstrated by the reaction of RNase A with azidocoumarin. The approach reported herein is fast, less labor-intensive and minimizes the risk for protein misfolding. Kinetically controlled labeling offers a high potential for addressing a broad range of native proteins and peptides in a site-selective fashion and complements the portfolio of recombinant techniques or chemoenzymatic approaches.

Site-specific PEGylation at histidine tags

The efficacy of protein-based medicines can be compromised by their rapid clearance from the blood circulatory system. Achieving optimal pharmacokinetics is a key requirement for the successful development of safe protein-based medicines. Protein PEGylation is a clinically proven strategy to increase the circulation half-life of protein-based medicines. One limitation of PEGylation is that there are few strategies that achieve site-specific conjugation of PEG to the protein. Here, we describe the covalent conjugation of PEG site-specifically to a polyhistidine tag (His-tag) on a protein. His-tag site-specific PEGylation was achieved with a domain antibody (dAb) that had a 6-histidine His-tag on the C-terminus (dAb-His(6)) and interferon α-2a (IFN) that had an 8-histidine His-tag on the N-terminus (His(8)-IFN). The site of PEGylation at the His-tag for both dAb-His(6)-PEG and PEG-His(8)-IFN was confirmed by digestion, chromatographic, and mass-spectral studies. A methionine was also inserted directly after the N-terminal His-tag in IFN to give His(8)Met-IFN. Cyanogen bromide digestion studies of PEG-His(8)Met-IFN were also consistent with PEGylation at the His-tag. By using increased stoichiometries of the PEGylation reagent, it was possible to conjugate two separate PEG molecules to the His-tag of both the dAb and IFN proteins. Stability studies followed by in vitro evaluation confirmed that these PEGylated proteins retained their biological activity. In vivo PK studies showed that all of the His-tag PEGylated samples displayed extended circulation half-lives. Together, our results indicate that site-specific, covalent PEG conjugation at a His-tag can be achieved and biological activity maintained with therapeutically relevant proteins.

A protein-polymer hybrid mediated by DNA

Protein-polymer hybrids (PPHs) represent an important and rapidly expanding class of biomaterials. Typically in these hybrids the linkage between the protein and the polymer is covalent. Here we describe a straightforward approach to a noncovalent PPH that is mediated by DNA. Although noncovalent, the DNA-mediated approach affords the highly specific pairing and assembly properties of DNA. To obtain the protein-DNA conjugate for assembly of the PPH, we report here the first direct copper catalyzed azide-alkyne cycloaddition-based protein-DNA conjugation. This significantly simplifies access to protein-DNA conjugates. The protein-DNA conjugate and partner polymer-DNA conjugate are readily assembled through annealing of the cDNA strands to obtain the PPH, the assembly of which was confirmed via dynamic light scattering and fluorescence spectroscopy.

PEGylation of antibody fragments for half-life extension

Antibody fragments (Fab's) represent important structure for creating new therapeutics. Compared to full antibodies Fab' fragments possess certain advantages, including higher mobility and tissue penetration, ability to bind antigen monovalently and lack of fragment crystallizable (Fc) region-mediated functions such as antibody-dependent cell mediated cytotoxicity (ADCC) or complement-dependent cytotoxicity (CDC). The main drawback for the use of Fab's in clinical applications is associated with their short half-life in vivo, which is a consequence of no longer having the Fc region. To exert meaningful clinical effects, the half-life of Fab's need to be extended, which has been achieved by postproduction chemical attachment of polyethylene glycol (PEG) chain to protein using PEGylation technology. The most suitable approach employs PEG-maleimide attachment to cysteines, either to the free hinge cysteine or to C-terminal cysteines involved in interchain disulfide linkage of the heavy and light chain. Hence, protocols for mono-PEGylation of Fab via free cysteine in the hinge region and di-PEGylation of Fab via interchain disulfide bridge are provided in this chapter.

Modulating antibody pharmacokinetics using hydrophilic polymers

INTRODUCTION

The use of hydrophilic polymers as a substitute for the Fc-domain in immuno- or non-immuno-based binding proteins is accelerating. Chemical PEGylation has led the way and is still the most advanced and clinically-approved approach. Hydrophilic polymers act by maintaining a flexible conformation and hydrogen bonding to a network of water molecules to acquire a larger hydrodynamic volume and apparent mass than their actual molecular mass suggest. The benefits are increased blood half-life and bioavailability, stability and reduced immunogenicity. In the case of PEG, there is also evidence of enhanced targeting and reduced side effects, but drawbacks include the fact that PEG is non-biodegradable.

AREAS COVERED

This report reviews the state of the art for antibody PEGylation in terms of approaches and effects. Additionally, non-biological (such as N-(2-hydroxypropyl)methacrylamide) and potentially superior biological alternatives (such as polysialylation) are described, ending with recombinant approaches (such as hydrophilic peptides and glyco-engineering), which promise to circumvent the need for chemical modification altogether.

EXPERT OPINION

The emergence of many small, antibody fragment-like mimics will drive the need for such technologies, and PEGylation is still the choice polymer due to its established use and track record. However, there will be a place for many alternative technologies if they can match the pharmacokinetics of PEG-conjugates and bring addition beneficial features such as easier production.

Poly(zwitterionic)protein conjugates offer increased stability without sacrificing binding affinity or bioactivity

Treatment with therapeutic proteins is an attractive approach to targeting a number of challenging diseases. Unfortunately, the native proteins themselves are often unstable in physiological conditions, reducing bioavailability and therefore increasing the dose that is required. Conjugation with poly(ethylene glycol) (PEG) is often used to increase stability, but this has a detrimental effect on bioactivity. Here, we introduce conjugation with zwitterionic polymers such as poly(carboxybetaine). We show that poly(carboxybetaine) conjugation improves stability in a manner similar to PEGylation, but that the new conjugates retain or even improve the binding affinity as a result of enhanced protein-substrate hydrophobic interactions. This chemistry opens a new avenue for the development of protein therapeutics by avoiding the need to compromise between stability and affinity.

State of the art in PEGylation: the great versatility achieved after forty years of research

In the recent years, protein PEGylation has become an established and highly refined technology by moving forward from initial simple random coupling approaches based on conjugation at the level of lysine ε-amino group. Nevertheless, amino PEGylation is still yielding important conjugates, currently in clinical practice, where the degree of homogeneity was improved by optimizing the reaction conditions and implementing the purification processes. However, the current research is mainly focused on methods of site-selective PEGylation that allow the obtainment of a single isomer, thus highly increasing the degree of homogeneity and the preservation of bioactivity. Protein N-terminus and free cysteines were the first sites exploited for selective PEGylation but currently further positions can be addressed thanks to approaches like bridging PEGylation (disulphide bridges), enzymatic PEGylation (glutamines and C-terminus) and glycoPEGylation (sites of O- and N-glycosylation or the glycans of a glycoprotein). Furthermore, by combining the tools of genetic engineering with specific PEGylation approaches, the polymer can be basically coupled at any position on the protein surface, owing to the substitution of a properly chosen amino acid in the sequence with a natural or unnatural amino acid bearing an orthogonal reactive group. On the other hand, PEGylation has not achieved the same success in the delivery of small drugs, despite the large interest and several studies in this field. Targeted conjugates and PEGs for combination therapy might represent the promising answers for the so far unmet needs of PEG as carrier of small drugs. This review presents a thorough panorama of recent advances in the field of PEGylation.